Seleprim is a biologically active supplement of 100 mcg Selenium.

Selenium is a micronutrient that is essential for the proper functioning of all organisms. Studies on the functions of selenium are rapidly developing. This element is a cofactor of many enzymes, for example, glutathione peroxidase or thioredoxin reductase. Insufficient supplementation of this element results in the increased risk of developing many chronic degenerative diseases. Selenium is important for the protection against oxidative stress, demonstrating the highest activity as a free radical scavenger and anti-cancer agent. In food, it is present in organic forms, as exemplified by selenomethionine and selenocysteine. In dietary supplementation, the inorganic forms of selenium (selenite and selenate) are used. Organic compounds are more easily absorbed by human organisms in comparison with inorganic compounds. Currently, selenium is considered an essential trace element of fundamental importance for human health. Extreme selenium deficiencies are widespread among people all over the world. Therefore, it is essential to supplement the deficiency of this micronutrient with selenium-enriched food or yeast cell biomass in the diet. [1]
Selenium is a commonly occurring element in nature. It can be found in the atmosphere, lithosphere, biosphere, and hydrosphere of the Earth. This element circulating in the environment initiates the process of weathering in rocks. Selenium is emitted into the atmosphere through volcanic gasses. Biomethylation of this element by microorganisms, decomposition of organic matter rich in this element, and so on contribute the most to the constant enrichment of the atmosphere with selenium. [2] 
Selenium is an essential bioelement that is necessary for the functioning of all organisms. The amount of this element present in nature and in the human organism is very diverse depending on the geographic region and diet. An optimal daily dose of this element is established at 55 µg and affects the normal course of biochemical and physiological processes. Selenium is present in the human organism in trace quantities. Serum selenium levels may differ among populations, depending on a number of factors, including, but not limited to, concentration of selenium in food. The concentration of this element in adult human blood serum depends on a person’s age. [3] 
The total amount of selenium in a human organism is ~3–20 mg. Skeletal muscles of the body are main organs containing ~46.9% of the total content of this element in humans, whereas kidneys contain only 4% of selenium. [4] The most commonly used indicator of “selenium status” is the determination of its concentration in serum, which is estimated at 60–120 ng/mL. The maximum selenium concentration is achieved in adulthood. The content of this element in serum is progressively decreasing in individuals >60 years of age. In the human body, deficiency of this element is observed when its amount in plasma is lower than 85 µg/L. Low selenium concentration in plasma is associated with 4- to 5-fold increased risk of prostate cancer. [5] 
Selenoxanthene is a biologically active substance containing selenium 9-phenyl-sym-octahydroselenoxanthene. This compound has antioxidant, bactericidal, radioprotective, antitumor, sedative, antispasmodic, analgesic, hypnotic effects and is used to prevent selenium deficiency in humans and animals, normalize thyroid function, increase the activity of antioxidant enzymes, and normalize the functions of the reproductive system. In particular, as an example of the normalizing effect of selenoxanthene on the functioning of the reproductive system, one can cite a method for correcting spermatogenesis in animals under conditions of chronic intoxication with natural gas, which reveals the possibility of selenoxanthene to improve the reproductive potential of males, due to the positive influence on spermogram indicators. [6]
Deficiency of selenium adversely affects the functioning of the cardiovascular system and can be a direct cause of myocardial infarction [7]. As a result of epidemiological studies, it was concluded that moderate deficiency of selenium in daily diet affects the development of diseases resulting from reduced immunity. Selenium deficiency in daily diet can adversely affect the functioning of the nervous system. Among individuals with selenium deficiency, development of depression, or intensification of anxiety is observed; Alzheimer’s disease is also developed. This element is considered to be crucial in reducing the virulence of HIV and in decreasing the progression to full-blown AIDS. Selenium deficiency in pregnant women negatively affects the development of the embryo. [8] 
It has been demonstrated that supplementation of fodder with selenium effectively protects against the increase of blood pressure in animals regularly exposed to heavy metal poisoning. Selenium supplementation reduces necrotic lesions in the testis and in animal fetus. In animals exposed to adverse effects of mercury compounds increased amounts of selenium reduce the formation of necrotic lesions in kidneys. Selenium plays a role of chelating agent of heavy metals by the formation of selenium–metal complexes. [9]
Also, selenium is a mineral that is important for thyroid hormone metabolism. In adults, the thyroid contains the highest amount of selenium per gram of tissue. Most known selenoproteins, such as glutathione peroxidase, are expressed in the thyroid and are involved in thyroid hormone metabolism, redox state regulation, and maintenance of cellular homeostasis. Some clinical studies have shown that lack of selenium will increase the prevalence of several kinds of thyroid diseases. Selenium treatment in patients with Graves’ orbitopathy has been shown to delay disease progression and improve the quality of life. Selenium supplementation in Hashimoto’s thyroiditis was associated with the decreased levels of anti-thyroid peroxidase antibody and improved thyroid ultrasound structure. In thyroid cancer, various selenium supplements have shown variable anticancer activity. [10]

References
[1] Kieliszek M. Selenium⁻Fascinating Microelement, Properties and Sources in Food. Molecules. 2019 Apr 3;24(7):1298. doi: 10.3390/molecules24071298. PMID: 30987088; PMCID: PMC6480557.
[2] Reich H.J., Hondal R.J. Why nature chose selenium? ACS Chem. Biol. 2016;11:821–841. doi: 10.1021/acschembio.6b00031.
[3] Post M., Lubiński W., Lubiński J., Krzystolik K., Baszuk P., Muszyńska M., Marciniak W. Serum selenium levels are associated with age-related cataract. Ann. Agric. Environ. Med. 2018;25:443–448. doi: 10.26444/aaem/90886.
[4] Lyons M.P., Papazyan T.T., Surai P.F. Selenium in food chain and animal nutrition: Lessons from nature. Asian Australas. J. Anim. Sci. 2007;20:1135–1155. doi: 10.5713/ajas.2007.1135.
[5] Zwolak I., Zaporowska H. Selenium interactions and toxicity: A review. Selenium interactions and toxicity. Cell Biol. Toxicol. 2012;28:31–46. doi: 10.1007/s10565-011-9203-9.
[6] BADIELLO R. et al. Radioluminescence of xanthene, thioxanthene and selenoxanthene following pulse irradiation // International Journal for Radiation Physics and Chemistry. - 1973. - V. 5. - No. 2. - P. 173-181
[7] Shahid M., Niazi N.K., Khalid S., Murtaza B., Bibi I., Rashid M.I. A critical review of selenium biogeochemical behavior in soil-plant system with an inference to human health. Environ. Pollut. 2018;234:915–934. doi: 10.1016/j.envpol.2017.12.019.
[8] Steinbrenner H., Sies H. Selenium homeostasis and antioxidant selenoproteins in brain: Implications for disorders in the central nervous system. Arch. Biochem. Biophys. 2013;536:152–157. doi: 10.1016/j.abb.2013.02.021.
[9] Żarczyńska K., Sobiech P., Radwińska J., Rękawek W. Effects of selenium on animal health. J. Elementol. 2013;18:329–340. doi: 10.5601/jelem.2013.18.2.12.
[10] Wang F, Li C, Li S, Cui L, Zhao J, Liao L. Selenium and thyroid diseases. Front Endocrinol (Lausanne). 2023 Mar 24;14:1133000. doi: 10.3389/fendo.2023.1133000. PMID: 37033262; PMCID: PMC10080082.